Method of producing lithium ion secondary battery and negative electrode material

ABSTRACT

A method of producing a lithium ion secondary battery disclosed here, includes a process of preparing an electrode body which includes a positive electrode and a negative electrode and in which the negative electrode contains a negative electrode material containing graphite having open pores and SiO 2  disposed in the open pores; a process of producing a battery assembly including the electrode body and a non-aqueous electrolytic solution containing LiPF 6  with a concentration of 1 mol/L or more; a process of initially charging the battery assembly; and a process of performing an aging treatment on the initially charged battery assembly in an environment of a temperature of 50° C. or higher.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2020-024584 filed on Feb. 17, 2020, incorporated herein by reference inits entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a method of producing a lithium ionsecondary battery. The present disclosure also relates to a negativeelectrode material of a lithium ion secondary battery.

2. Description of Related Art

In recent years, lithium ion secondary batteries have been suitably usedfor portable power supplies of computers, mobile terminals, and the likeand power supplies for driving vehicles such as electric vehicles (EV),hybrid vehicles (HV), and plug-in hybrid vehicles (PHV).

With the spread of lithium ion secondary batteries, higher performanceis desired. Regarding one technology for improving performance oflithium ion secondary batteries, a technology in which, in order toimprove cycle characteristics and storage characteristics, lithiumdifluorophosphate (LiPO₂F₂) is added to a non-aqueous electrolyticsolution, and a coating derived from lithium difluorophosphate is formedon an electrode is known (for example, refer to Japanese UnexaminedPatent Application Publication No. 2005-219994 (JP 2005-219994 A)). Itis disclosed in JP 2005-219994 A that, when lithium hexafluorophosphate(LiPF₆), which is widely used as a supporting salt (electrolyte salt) ina non-aqueous electrolytic solution, is reacted with silicon dioxide(SiO₂) in a non-aqueous solvent, a non-aqueous electrolytic solutioncontaining lithium difluorophosphate can be easily produced.

SUMMARY

However, according to studies by the inventors, it has been found that,when a non-aqueous electrolytic solution contains lithiumdifluorophosphate, lowering of the resistance of a lithium ion secondarybattery is insufficient. Specifically, lithium difluorophosphate is acomponent that can contribute to lowering the resistance of the lithiumion secondary battery. However, it is found that, when the non-aqueouselectrolytic solution contains lithium difluorophosphate with a highconcentration, the lithium ion conductivity of the non-aqueouselectrolytic solution decreases, and as a result, the resistanceactually becomes higher.

Therefore, the present disclosure provides a method by which it ispossible to produce a lithium ion secondary battery having lowresistance.

The method of producing a lithium ion secondary battery disclosed hereincludes a process of preparing an electrode body which includes apositive electrode and a negative electrode and in which the negativeelectrode contains a negative electrode material containing graphitehaving open pores and SiO₂ disposed in the open pores; a process ofproducing a battery assembly including the electrode body and anon-aqueous electrolytic solution containing LiPF₆ with a concentrationof 1 mol/L or more; a process of initially charging the batteryassembly; and a process of performing an aging treatment on theinitially charged battery assembly in an environment of a temperature of50° C. or higher.

In such a configuration, it is possible to produce a lithium ionsecondary battery having low resistance.

In a preferable aspect of the method of producing a lithium ionsecondary battery disclosed here, the porosity of the graphite is 5% ormore and 25% or less.

In such a configuration, it is possible to further reduce the batteryresistance while maintaining the strength of graphite.

In a preferable aspect of the method of producing a lithium ionsecondary battery disclosed here, a mass ratio of SiO₂ to the graphiteis 0.5 mass % or more and 10 mass % or less.

In such a configuration, it is possible to further reduce the batteryresistance.

In another aspect, regarding a material that can be used in theproduction method, a negative electrode material of a lithium ionsecondary battery including graphite having open pores and SiO₂ disposedin the open pores is disclosed.

In a preferable aspect of the negative electrode material disclosedhere, the porosity of the graphite is 5% or more and 25% or less.

In a preferable aspect of the negative electrode material disclosedhere, a mass ratio of SiO₂ to the graphite is 0.5 mass % or more and 10mass % or less.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like signs denote likeelements, and wherein:

FIG. 1 is a schematic exploded view illustrating a configuration of anelectrode body prepared according to one embodiment of the presentdisclosure; and

FIG. 2 is a cross-sectional view schematically showing a configurationof a battery assembly produced according to one embodiment of thepresent disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the present disclosure will be described below withreference to the drawings. Here, components other than those mentionedin this specification that are necessary for implementing the presentdisclosure can be recognized by those skilled in the art as designmatters based on the related art in the field. The present disclosurecan be implemented based on content disclosed in this specification andcommon general technical knowledge in the field. In addition, membersand portions having the same functions are denoted by the same referencenumerals in the following drawings, and redundant descriptions thereofwill be omitted. In addition, the sizes (a length, a width, a thicknessand the like) in the drawings do not reflect actual sizes.

Here, the term “secondary battery” in this specification refers to apower storage device that can be repeatedly charged and discharged, andincludes a so-called storage battery and a storage element such as anelectric double layer capacitor.

In addition, the term “lithium ion secondary battery” in thisspecification refers to a secondary battery in which lithium ions areused as charge carriers, and charging and discharging are realized bymovement of charges according to lithium ions between positive andnegative electrodes.

Hereinafter, although the present disclosure will be described below indetail using a method of producing a flat rectangular lithium ionsecondary battery including a wound electrode body as an example, thepresent disclosure is not intended to be limited to those described inthese embodiments.

The method of producing a lithium ion secondary battery according to thepresent embodiment includes the following processes (A) to (D).

Process (A) of preparing an electrode body which includes a positiveelectrode and a negative electrode and in which the negative electrodecontains a negative electrode material containing graphite having openpores and SiO₂ disposed in the open pores;

Process (B) of producing a battery assembly including the electrode bodyand a non-aqueous electrolytic solution containing LiPF₆ with aconcentration of 1 mol/L or more;

Process (C) of initially charging the battery assembly; and

Process (D) of performing an aging treatment on the initially chargedbattery assembly in an environment of a temperature of 50° C. or higher.

FIG. 1 is a schematic exploded view illustrating a configuration of anelectrode body prepared according to the present embodiment. FIG. 2 is across-sectional view schematically showing a configuration of a batteryassembly produced according to the present embodiment.

Process (A): Electrode Body Preparation Process

In the process (A), an electrode body 20 including a positive electrode50 and a negative electrode 60 is prepared.

As shown in FIG. 1 and FIG. 2 , the positive electrode 50 has typicallya sheet form, and includes a positive electrode current collector 52,and a positive electrode active material layer 54 provided on onesurface or both surfaces of the positive electrode current collector 52.In the shown example, the positive electrode 50 includes a positiveelectrode active material layer non-forming part 52 a in which thepositive electrode current collector 52 is exposed at an end thereof.The positive electrode active material layer non-forming part 52 afunctions as a current collecting part, but the configuration of thecurrent collecting part is not limited thereto.

Regarding the positive electrode current collector 52, an aluminum foilor the like can be used.

The positive electrode active material layer 54 contains a positiveelectrode active material. Regarding the positive electrode activematerial, a known positive electrode active material used for a lithiumsecondary battery may be used. Examples of positive electrode activematerials include lithium-containing transition metal oxides such as alithium nickel composite oxide, a lithium cobalt composite oxide, alithium manganese composite oxide, a lithium nickel manganese compositeoxide (for example, LiNi_(0.5)Mn_(1.5)O₄), and a lithium nickelmanganese cobalt composite oxide (for example,LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂); and lithium transition metal phosphatecompounds such as LiFePO₄. Among these, a lithium nickel manganesecobalt composite oxide is preferable.

The content of the positive electrode active material is notparticularly limited, and is preferably 70 mass % or more and morepreferably 80 mass % or more in the positive electrode active materiallayer 54 (that is, with respect to a total mass of the positiveelectrode active material layer 54).

The positive electrode active material layer 54 may contain componentsother than the positive electrode active material, for example,trilithium phosphate, a conductive material, and a binder. Regarding theconductive material, for example, carbon black such as acetylene black(AB) and other carbon materials (for example, graphite) can be suitablyused. Regarding the binder, for example, polyvinylidene fluoride (PVdF)can be used.

The content of trilithium phosphate in the positive electrode activematerial layer 54 is not particularly limited, and is preferably 1 mass% or more and 15 mass % or less and more preferably 2 mass % or more and12 mass % or less.

The content of the conductive material in the positive electrode activematerial layer 54 is not particularly limited, and is preferably 1 mass% or more and 15 mass % or less and more preferably 3 mass % or more and13 mass % or less.

The content of the binder in the positive electrode active materiallayer 54 is not particularly limited, and is preferably 1 mass % or moreand 15 mass % or less, and more preferably 1.5 mass % or more and 10mass % or less.

The positive electrode 50 can be produced according to a known method.For example, the positive electrode 50 can be produced by preparing apositive electrode paste containing components constituting the positiveelectrode active material layer 54 and a solvent (for example,N-methylpyrrolidone), applying the positive electrode paste to thepositive electrode current collector 52, and drying it. The positiveelectrode active material layer 54 formed by drying may be additionallysubjected to press processing.

As shown in FIG. 1 and FIG. 2 , the negative electrode 60 has typicallya sheet form, and includes a negative electrode current collector 62 anda negative electrode active material layer 64 provided on one surface orboth surfaces of the negative electrode current collector 62. In theshown example, the negative electrode 60 includes a negative electrodeactive material layer non-forming part 62 a in which the negativeelectrode current collector 62 is exposed at an end thereof. Thenegative electrode active material layer non-forming part 62 a functionsas a current collecting part, but the configuration of the currentcollecting part is not limited thereto.

Regarding the negative electrode current collector 62, a copper foil orthe like can be used.

The negative electrode 60 contains, particularly, in the negativeelectrode active material layer 64, a negative electrode materialcontaining graphite having open pores and SiO₂ disposed in the openpores.

Graphite having open pores is a negative electrode active material.Here, the open pores are pores that are connected to outside air. Thegraphite may be natural graphite or a modified product thereof or may beartificial graphite.

The primary particle size of SiO₂ is generally smaller than the openingdiameter of the open pores. The opening diameter of the open pores andthe primary particle size of SiO₂ can be measured using an electronmicroscope.

In the negative electrode material, a mass ratio of SiO₂ to graphite isnot particularly limited. In the negative electrode material, when theamount of SiO₂ is too small, a resistance reduction effect becomes weak,and in order to obtain a strong resistance reduction effect, a timerequired for an aging process becomes very long, and there is a risk ofproduction efficiency decreasing. Therefore, in order to obtain astronger resistance reduction effect, the mass ratio of SiO₂ to graphiteis preferably 0.5 mass % or more and more preferably 1.5 mass % or more.In addition, when the amount of SiO₂ is too large, this may causeaggregation of SiO₂ and there is a risk of an effect of loweringresistance being weakened. Therefore, in order to obtain a strongerresistance reduction effect, the mass ratio of SiO₂ to graphite ispreferably 10 mass % or less, and more preferably 5 mass % or less.

In the negative electrode material, the porosity of graphite is notparticularly limited. In the negative electrode material, when theporosity is too small, the amount of SiO₂ that can be retained is small,and as a result, there is a risk of a resistance reduction effect beingweakened. Therefore, in order to obtain a stronger resistance reductioneffect, the porosity of graphite is preferably 5% or more and morepreferably 12.5% or more. When the porosity of graphite is too large,there is a risk of the strength of graphite decreasing. When thestrength of graphite is too low, the graphite may be damaged(particularly, the open pores may be blocked) when the negativeelectrode active material layer 64 is subjected to press processing.Therefore, the porosity of graphite is preferably 25% or less and morepreferably 20% or less.

Here, the porosity of graphite can be determined, for example, bymeasuring an apparent density using a mercury intrusion porosimeter andmeasuring a true density with a helium pycnometer and calculating“porosity (%)=(1−apparent density/true density)×100.”

Since the negative electrode material includes a negative electrodematerial containing graphite having open pores and SiO₂ disposed in theopen pores, it has generally a particle form.

The negative electrode material can be produced, for example, asfollows.

(1) Graphite having open pores and SiO₂ having a primary particle size(typically less than 100 nm) smaller than the opening diameter of theopen pores is prepared. These are stirred and mixed at a predeterminedmass ratio in a dispersion medium (for example, alcohols such asethanol) to prepare a dispersion solution. While stirring using anevaporator or the like, the dispersion medium is removed from thedispersion solution under a reduced pressure. The residual dispersionmedium is removed from the obtained mixture by performing drying such asheating.(2) Graphite having open pores and SiO₂ having a primary particle size(typically less than 100 nm) smaller than the opening diameter of theopen pores are prepared. These are stirred and mixed at a predeterminedmass ratio in a dispersion medium (for example, alcohols such asethanol) to prepare a dispersion solution. The dispersion solution isput into an autoclave, and heated, for example, at a temperature of 80°C. to 120° C. (particularly about 100° C.) and for example, for 2 hoursto 12 hours (particularly about 6 hours). After cooling, solidcomponents are filtered off and the filtrate is washed and dried.

The content of the negative electrode material in the negative electrodeactive material layer 64 is not particularly limited, and is preferably90 mass % or more and more preferably 94 mass % or more.

The negative electrode active material layer 64 may include componentsother than the negative electrode material, for example, a binder and athickener.

Regarding the binder, for example styrene butadiene rubber (SBR) can beused. Regarding the thickener, for example, carboxymethyl cellulose(CMC) can be used.

The content of the binder in the negative electrode active materiallayer 64 is not particularly limited, and is preferably 0.1 mass % ormore and 8 mass % or less and more preferably 0.5 mass % or more and 3mass % or less.

The content of the thickener in the negative electrode active materiallayer 64 is not particularly limited, and is preferably 0.3 mass % ormore and 3 mass % or less and more preferably 0.5 mass % or more and 2mass % or less.

The negative electrode 60 can be produced according to a known method.For example, the negative electrode 60 can be produced by preparing anegative electrode paste containing components constituting the negativeelectrode active material layer 64 and a solvent (for example, water),applying the negative electrode paste to the negative electrode currentcollector 62, and drying it. The negative electrode active materiallayer 64 formed by drying may be additionally subjected to pressprocessing.

In addition, the electrode body 20 includes generally a separator 70that insulates the positive electrode 50 and the negative electrode 60from each other. Regarding the separator 70, for example, a porous sheet(film) made of a resin such as polyethylene (PE), polypropylene (PP),polyester, cellulose, or polyamide may be exemplified. Such a poroussheet may have a single-layer structure or a structure in which two ormore layers are laminated (for example, a three-layer structure in whicha PP layer is laminated on both surfaces of a PE layer). A heatresistant layer (HRL) may be provided on the surface of the separator70.

The separator 70 can be produced according to a known method.

In the process (A), the electrode body 20 is produced using the positiveelectrode 50, the negative electrode 60, and the separator 70 describedabove. In the present embodiment, the electrode body 20 is a woundelectrode body. The electrode body 20 can be produced according to aknown method. Specifically, for example, the positive electrode 50 andthe negative electrode 60 are superimposed with two separators 70therebetween to produce a laminate, and the laminate is wound in alongitudinal direction. In this case, the positive electrode activematerial layer non-forming part 52 a and the negative electrode activematerial layer non-forming part 62 a are wound so that they protrudeoutward from both ends of the electrode body 20 in a winding axisdirection. In this manner, the electrode body 20 can be produced. Here,in order to obtain a flat wound electrode body shown in FIG. 1 , thelaminate may be wound in a flat shape, or a cylindrical wound body ofthe laminate is produced first and this may be pressed in a side surfacedirection and squeezed.

Process (B): Battery Assembly Production Process

In the process (B), a battery assembly 90 including the electrode body20 prepared above and a non-aqueous electrolytic solution 80 containingLiPF₆ with a concentration of 1 mol/L or more is produced.

The non-aqueous electrolytic solution 80 typically contains anon-aqueous solvent and LiPF₆ which is a supporting salt (electrolytesalt) with a concentration of 1 mol/L or more.

The upper limit of the concentration of LiPF₆ in the non-aqueouselectrolytic solution 80 is, for example, 2.0 mol/L or less. In order toobtain a particularly strong battery resistance reduction effect, theconcentration of LiPF₆ is preferably 1.3 mol/L or more and 1.7 mol/L orless.

Regarding the non-aqueous solvent, organic solvents such as variouscarbonates, ethers, esters, nitriles, sulfones, and lactones which aregenerally used in the electrolytic solution of a lithium ion secondarybattery can be used without particular limitation. Among these,preferable examples of carbonates include ethylene carbonate (EC),propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate(DMC), ethyl methyl carbonate (EMC), monofluoro ethylene carbonate(MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate(TFDMC). Such non-aqueous solvents may be used alone or two or morethereof may be appropriately used in combination.

The non-aqueous electrolytic solution 80 may contain lithiumdifluorophosphate (LiPO₂F₂). In this case, the concentration of lithiumdifluorophosphate in the non-aqueous electrolytic solution 80 is, forexample, 0.005 mol/L or more and 0.1 mol/L or less, and preferably 0.03mol/L or more and 0.07 mol/L or less.

The non-aqueous electrolytic solution 80 may contain components otherthan the above components, for example, various additives: a gasgenerating agent such as biphenyl (BP) and cyclohexylbenzene (CHB); acoating forming agent; and a thickener, as long as the effects of thepresent disclosure are not significantly impaired.

In order to produce the battery assembly 90, a battery case 30 isprepared. As shown in FIG. 2 , the battery case 30 includes a case bodyhaving an opening and a lid that blocks the opening. A thin safety valve36 that is set to release an internal pressure when the internalpressure of the battery case 30 increases to a predetermined level orhigher is provided in the lid of the battery case 30. In addition, aninlet (not shown) through which the non-aqueous electrolytic solution 80is injected is provided in the lid of the battery case 30. Examples ofmaterials of the battery case 30 include aluminum and an aluminum alloy.

Next, the electrode body 20 is accommodated in the battery case 30according to a known method. As shown in FIG. 2 , a positive electrodeterminal 42 and a positive electrode current collecting plate 42 a, anda negative electrode terminal 44 and a negative electrode currentcollecting plate 44 a are attached to the lid of the battery case 30.The positive electrode current collecting plate 42 a and the negativeelectrode current collecting plate 44 a are welded to the positiveelectrode current collector 52 and the negative electrode currentcollector 62 (that is, the positive electrode active material layernon-forming part 52 a and the negative electrode active material layernon-forming part 62 a) exposed at the end of the electrode body 20.Then, the electrode body 20 is accommodated inside a main body of thebattery case 30 through the opening, and the case body of the batterycase 30 and the lid are welded.

Here, the form of the battery case 30 is not limited to the above form.For example, regarding the battery case 30, a battery case made of alaminate film (so called a laminate case) may be used.

Then, the electrolytic solution 80 is injected from the inlet at the lidof the battery case 30. After the electrolytic solution 80 is injected,the inlet is sealed, and thereby the battery assembly 90 can beobtained.

Here, in FIG. 2 , an exact amount of the non-aqueous electrolyticsolution 80 is not shown.

Here, “battery assembly” in this specification refers to an assembly ofbattery components required for an initial charging process and an agingprocess to be described below. Therefore, for example, the lid of thebattery case 30 or the inlet which is included in the lid for anon-aqueous electrolytic solution may be used before sealing or aftersealing.

Process (C): Initial Charging Process

Initial charging can be performed according to a known method.

Specifically, for example, an external power supply is connected betweenthe positive electrode terminal 42 and the negative electrode terminal44 of the battery assembly produced above, and charging is performed atroom temperature (generally, 25° C.±5° C.) until a voltage between thepositive electrode terminal 42 and the negative electrode terminal 44reaches a predetermined value. This charging may be constant currentcharging (CC charging) or constant current and constant voltage charging(CC-CV charging). For example, this charging can be performed accordingto constant current and constant voltage charging (CC-CV charging) inwhich charging is performed with a constant current of about 0.1 C to 10C until a voltage between terminals reaches a predetermined value fromwhen charging starts, and charging is then performed with a constantvoltage until a state of charge (SOC) reaches about 60% to 100%(preferably about 80% to 100%).

Process (D): Aging Process

In the process (D), the battery assembly is left in an environment of atemperature of 50° C. or higher. When the process (D) is performed usingthe negative electrode material containing SiO₂ in the open pores, andLiPF₆ with a specific concentration as an electrolyte salt, it ispossible to obtain a lithium ion secondary battery 100 having lowresistance. Specifically, in the process (D), LiPF₆ in the non-aqueouselectrolytic solution 80 reacts with SiO₂ in the negative electrodematerial, and lithium difluorophosphate (LiPO₂F₂) with a highconcentration is generated on the surface of graphite. The generatedLiPO₂F₂ forms a coating having low resistance on the surface of graphiteas a negative electrode active material. That is, in the related art,when LiPO₂F₂ with a high concentration is contained in the non-aqueouselectrolytic solution in order to reduce the resistance, the lithium ionconductivity of the non-aqueous electrolytic solution decreases and theresistance actually becomes higher. On the other hand, according to theproduction method of the present embodiment, since LiPO₂F₂ with a highconcentration is locally generated on the surface of graphite on whichthe coating is formed, and the coating having low resistance can beformed on the surface of graphite, it is possible to form a coatingderived from LiPO₂F₂ and having low resistance without decreasing thelithium ion conductivity.

The process (D) can be performed according to a known method. Forexample, the process (D) can be performed by a method in which theinitially charged battery assembly 90 is left in a constant temperaturechamber of which the temperature is set to 50° C. or higher.

The aging temperature is, for example, 50° C. or higher and 80° C. orlower, and typically 50° C. or higher and 75° C. or lower.

The aging time may be appropriately determined according to thetemperature, the amount of SiO₂ contained in the negative electrodematerial, the concentration of LiPF₆, and the like. The aging time is,for example, 12 hours or longer, and preferably 20 hours or longer. Inaddition, the aging time is, for example, 240 hours or shorter,preferably 120 hours or shorter, and more preferably 72 hours orshorter.

In the process (D), the SOC of the battery assembly is preferably 60% ormore and 100% or less and more preferably 80% or more and 100% or less.

The lithium ion secondary battery 100 obtained as described above has anadvantage of low resistance. In addition, it also has excellent storagecharacteristics and cycle characteristics. The obtained lithium ionsecondary battery 100 can be used for various applications. Examples ofsuitable applications include power supplies for driving mounted invehicles such as an electric vehicle (EV), a hybrid vehicle (HV), and aplug-in hybrid vehicle (PHV). The lithium ion secondary battery 100 canalso be used typically in the form of an assembled battery in which aplurality of batteries are connected in series and/or in parallel.

Here, the method of producing a rectangular lithium ion secondarybattery including a flat wound electrode body has been described as anexample. However, the battery produced according to the productionmethod of the present embodiment is not limited thereto, and may be alithium ion secondary battery including a laminated electrode body, acoin type lithium ion secondary battery, a cylindrical lithium ionsecondary battery, a laminate lithium ion secondary battery, or thelike.

While examples of the present disclosure will be described below, thepresent disclosure is not intended to be limited to those described inthese examples. Preparation of negative electrode material

50 g of graphite having a particle size (D50) of 10 μm and a porosityshown in Table 1 and SiO₂ having a particle size (D50) of 80 nm weresufficiently stirred and mixed in 100 mL of ethanol to obtain adispersion solution. A mass ratio of SiO₂ to graphite was a value shownin Table 1. Ethanol was removed using an evaporator from the dispersionsolution while stirring under a reduced pressure. The obtained solidcomponent was dried at 100° C. for 12 hours, and the residual ethanolwas removed.

In this manner, negative electrode materials of test examples wereobtained.

Production of Battery Assembly

LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ (LNCM) as a positive electrode activematerial, acetylene black (AB) as a conductive material, andpolyvinylidene fluoride (PVdF) as a binder at a mass ratio ofLNCM:AB:PVdF=92:5:3 were mixed with N-methylpyrrolidone (NMP) to preparea slurry for forming a positive electrode active material layer. Thisslurry was applied to both surfaces of an elongated aluminum foil with athickness of 15 μm in a width of 100 mm, and dried, and thenroll-pressed to have a predetermined thickness, and thereby a positiveelectrode sheet was produced.

A negative electrode material, styrene butadiene rubber (SBR) as abinder, and carboxymethyl cellulose (CMC) as a thickener at a mass ratioof C:SBR:CMC=98:1:1 were mixed with deionized water to prepare a slurryfor forming a negative electrode active material layer. This slurry wasapplied in a belt shape to both surfaces of an elongated copper foilhaving a thickness of 10 μm, dried, and then roll-pressed, and thereby anegative electrode sheet was produced.

In addition, a separator sheet in which a ceramic layer (heat resistantlayer) having a thickness of 4 μm was formed on the surface of a porouspolyolefin sheet having a three-layer structure of PP/PE/PP and having athickness of 24 μm was prepared. The produced positive electrode sheetand negative electrode sheet were made to face each other with theseparator sheet therebetween to produce an electrode body. Here, theheat resistant layer of the separator sheet was made to face thenegative electrode.

An electrolyte salt shown in Table 1 and with a concentration shown inTable 1, and LiPO₂F₂ with a concentration of 0.05 mol/L were dissolvedin a mixed solvent containing ethylene carbonate (EC), dimethylcarbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio ofEC:DMC:EMC=3:3:4, and thereby a non-aqueous electrolytic solution wasprepared.

An aluminum case including a lid having a liquid injection port and acase body was prepared.

An electrode terminal and a current collecting plate were attached tothe lid. Then, the produced electrode body and the current collectingplate were bonded by welding. The electrode body bonded to the lid inthis manner was inserted into the case body, and the lid and the casebody were welded.

A predetermined amount of the prepared non-aqueous electrolytic solutionwas injected from the liquid injection port, and a sealing screw wasfastened to the liquid injection port and sealing was performed.

The non-aqueous electrolytic solution was left for a predetermined timeso that it was impregnated into the electrode body, and thereby abattery assembly was obtained.

Initial Charging and Aging Treatment

Initial charging was performed according to a constant current-constantvoltage method. The produced battery assembly was subjected to constantcurrent charging until the voltage reached 4.1 V at a current value of ⅓C, and constant voltage charging was then performed until the currentvalue reached 1/50 C, and the battery was fully charged.

Then, an aging treatment was performed in a thermostatic chamber at 50°C. for 20 hours, and lithium ion secondary batteries of test exampleswere completed.

However, in Test Example 2, a lithium ion secondary battery wascompleted by performing only initial charging without performing anaging treatment.

Evaluation of Resistance

The SOC of the lithium ion secondary batteries obtained above wasadjusted to 56%, and the batteries were left in an environment of atemperature of −10° C. Charging was performed at this temperature and ata predetermined current value for 30 seconds, and a voltage changeamount ΔV was determined. A resistance value was calculated by dividingthe voltage change amount ΔV by the current value. A ratio of theresistance value of other test examples to 100 which was the resistancevalue of Test Example 1 was determined. The results are shown in Table1.

TABLE 1 Negative electrode material Ratio Porosity of of SiO₂Electrolyte salt graphite (mass Concentration Aging Resistance (%) %)Type (mol/L) treatment ratio Test 15 0 LiPF₆ 1.5 Yes 100 Example 1 Test15 3 LiPF₆ 1.5 No 102 Example 2 Test 15 3 LiBF₄ 1.5 Yes 108 Example 3Test 15 3 LiPF₆ 1.5 Yes 94 Example 4 Test 15 3 LiPF₆ 0.9 Yes 103 Example5 Test 15 3 LiPF₆ 1 Yes 95 Example 6 Test 15 0.5 LiPF₆ 1.5 Yes 97Example 7 Test 15 10 LiPF₆ 1.5 Yes 97 Example 8 Test 0 3 LiPF₆ 1.5 Yes99 Example 9 Test 5 3 LiPF₆ 1.5 Yes 96 Example 10 Test 10 3 LiPF₆ 1.5Yes 97 Example 11 Test 25 3 LiPF₆ 1.5 Yes 96 Example 12

Test Examples 4, 6 to 8, and 10 to 12 were test examples within therange of the production method according to the present embodimentdescribed above.

Based on the results of Test Examples 1 to 6 and 9, it can be understoodthat, when graphite had open pores, the graphite contained SiO₂ in theopen pores, the electrolyte salt was LiPF₆, the concentration thereofwas 1 mol/L or more, and an aging process was performed, it was possibleto reduce the resistance of the lithium ion secondary battery.

In addition, based on the results of Test Examples 4, 7, and 8, it canbe understood that, even if the ratio of the SiO₂ content to graphitechanged, a resistance reduction effect was obtained.

Based on the results of Test Examples 4 and 10 to 12, it can beunderstood that, even if the porosity of graphite changed, a resistancereduction effect was obtained.

In addition, the same experiment was performed using Li₄Ti₅O₁₂ or SiO inplace of graphite as a negative electrode active material, but thebattery resistance was higher than in Test Example 1. This is thought tobe caused by the fact that, when Li₄Ti₅O₁₂ was used, the potential wasnot suitable for forming a coating derived from LiPO₂F₂. In addition, itis also thought to be caused by the fact that, when SiO was used, anamount of expansion and contraction of SiO due to charging anddischarging increased, and thereby a coating derived from LiPO₂F₂deteriorated.

As described above, according to the method of producing a lithium ionsecondary battery disclosed here, it is possible to produce a lithiumion secondary battery having low resistance.

While specific examples of the present disclosure have been describedabove in detail, these are only examples, and do not limit the scope ofthe claims. The technology described in the scope of the claims includesvarious modifications and alternations of the specific examplesexemplified above.

What is claimed is:
 1. A method of producing a lithium ion secondarybattery, comprising: preparing an electrode body which includes apositive electrode and a negative electrode and in which the negativeelectrode contains a negative electrode material containing porousgraphite particles having open pores within the graphite and SiO₂disposed in the open pores; producing a battery assembly including theelectrode body and a non-aqueous electrolytic solution containing LiPF₆with a concentration of 1 mol/L or more; initially charging the batteryassembly; and performing an aging treatment on the initially chargedbattery assembly in an environment of a temperature of 50° C. or higher.2. The production method according to claim 1, wherein the porosity ofthe graphite is 5% or more and 25% or less.
 3. The production methodaccording to claim 1, wherein a mass ratio of SiO₂ to the graphite is0.5 mass % or more and 10 mass % or less.
 4. A negative electrodematerial of a lithium ion secondary battery, comprising porous graphiteparticles having open pores within the graphite and SiO₂ disposed in theopen pores.
 5. The negative electrode material according to claim 4,wherein the porosity of the graphite is 5% or more and 25% or less. 6.The negative electrode material according to claim 4, wherein a massratio of SiO₂ to the graphite is 0.5 mass % or more and 10 mass % orless.
 7. The production method according to claim 1, wherein the SiO₂ isin particle form and a primary particle size of the SiO₂ is smaller thanan opening diameter of the open pores.
 8. The production methodaccording to claim 7, wherein the primary particle size of the SiO₂ isless than 100 nm.
 9. The negative electrode material according to claim4, wherein the SiO₂ is in particle form and a primary particle size ofthe SiO₂ is smaller than an opening diameter of the open pores.
 10. Thenegative electrode material according to claim 9, wherein the primaryparticle size of the SiO₂ is less than 100 nm.